1. Field of the Invention
The present invention relates to an observation optical system to be used as a viewfinder optical system of a still camera or video camera or to be used in binoculars, etc.
2. Description of the Relating Art
Previously, various methods of suppressing reflection in an optical system have been proposed. The following anti-reflection methods are known as methods for obtaining a good field of view in a viewfinder optical system of a still camera, video camera, etc.
First, as an arrangement, in which an anti-reflection film of MgF2, SiO2, etc. is applied to a predetermined optical surface in order to suppress the occurrence of ghosts due to surface reflection by a lens surface, Japanese Laid-Open Patent Publication No. 1995-77730 (corresponding to U.S. Pat. No. 5,442,481) proposes a viewfinder optical system in which an anti-reflection film is applied to the surface of an objective lens.
Japanese Laid-Open Patent Publication No. 1996-179400 proposes a viewfinder optical system, with which multiple reflection is suppressed by adding an anti-reflection film to either of two prism surfaces that sandwich a minute air interval.
Though an anti-reflection film is known to provide the effects of suppressing Fresnel reflection at a lens surface and thereby improving the transmittance of the lens, with plastic materials used in prior viewfinder optical systems, the surface reflection was of a negligible level due to the low refractive index and the tolerance of the human eye to ghost light.
However, as cameras are being made more compact in recent years, compact viewfinder optical systems are coming in demanded. For compact size, the lenses that make up a viewfinder optical system must be positioned closer to each other. Also in order to correct the various aberrations that tend to be generated as the size is made more compact, there is a tendency for an increased number of objective lenses and ocular lenses.
The decrease of transmittance due to the Fresnel reflection at lens surfaces becomes non-negligible as the number of lenses increase.
Also due to the lenses being positioned closer to each other, so-called surface reflection ghost, which is ghost caused by the light rays that are reflected among the lens surfaces, occurs. Though the lenses can be positioned at adequate lens intervals in order to suppress the occurrence of surface reflection ghost, this will prevent the requirement of the compact size of the viewfinder optical system or camera from being met.
An anti-reflection film may be applied to the optical surfaces in order to suppress reflection by the lens surfaces while maintaining compact size. As indicated in Japanese Laid-Open Patent Publication No. 1995-77730 (corresponding to U.S. Pat. No. 5,442,481) and Japanese Laid-Open Patent Publication No. 1996-179400, when an anti-reflection film is applied to a lens surface of a plastic lens and this anti-reflection film is to be a single-layer film made from MgF2, SiO2, etc. are generally used. However, since these materials are weak in adhesion to plastic, in many actual cases, a plastic film for improving the adhesion is formed on the surface of the plastic lens in advance, and the abovementioned anti-reflection film is applied above this plastic film.
In some cases, a protective film layer for improving wear resistance, surface hardness, etc. is also be provided additionally.
Also, in forming an above-described anti-reflection film on an optical member made of a plastic material, since the optical member may deform due to heating, vapor deposition at high temperature could not be performed and this presented a disadvantage in terms of durability.
Thus with the prior-art methods of suppressing reflection, even if inexpensive plastic materials are used in optical members, the provision of coating lead to increases in cost as well as an increase in manufacturing processes, and an anti-reflection structure of stable quality could not be obtained in some cases.
An anti-reflection structure for a viewfinder optical system or other observation optical system is thus required which will not lead to increase size or cost and to enable stable quality to be obtained without an increase in manufacturing processes.